This invention relates to a method and apparatus for detecting the breakage of glass.
Detecting glass breakage is important in securing buildings from illegal entry. It is well known that illegal entry into buildings can be obtained by breaking the glass of a window and reaching in to open the window. Illegal entry may also be obtained by breaking glass panels on or around a door and then reaching in to unlock the door and thus gain entry. The entire window or glass doors may be shattered in order to gain illegal entry. Thus, there is considerable interest in providing security systems for these buildings with a means to detect the breaking of glass.
Glass breakage detectors are known in the art. Vibrational type glass breakage detectors are either installed on the frame of the glass or on the glass itself. These type of detectors are not easy to install because they must receive sufficient energy when impact is applied to the glass to produce an alarm but not be overly sensitive to other vibrations which may be transmitted through the structure or be airborne transmitted. Furthermore, these sensors are difficult to test because a true test involves shattering the glass which is impractical. Thus, adjusting the sensitivity of these devices can be difficult and require repeat adjustments if false alarms are a problem. Glass mounted detectors of this type are limited to a single pane and thus one sensor is required for each pane in multi-partitioned glass.
Sound discriminator type sensors are much easier to install but are prone to false alarms because of the fact that the useful frequencies and energy levels of airborne-generated sounds of breaking glass are also commonly generated by many sources in a typical home or business such as radios, human speech, the moving of furniture, normal handling of desk components, files, dishes, pots, pans, drinking glasses or similar articles.
More recently sound discriminators which incorporate two transducers have become available. Each of these transducers respond to one of the two major acoustical energy components associated with breaking glass. The first transducer is generally an ordinary microphone which is intended to respond primarily to the higher frequencies of the airborne-generated component of breaking glass. The other transducer is quite different and is specially designed to respond to the lower-frequency structurally-generated component. By utilizing two transducers, each detecting a different component of breaking glass, these devices minimize the probability of false alarms without sacrificing effective glass breakage detection when it truly occurs within range of the detector.
U.S. Pat. No. 4,195,286 which issued on Mar. 25, 1980 to Aaron Galvin discloses the principle of using two or more transducers or sensors for the purpose of providing redundancy in an alarm system to reduce the probability of false alarms. In this system, the outputs of the two transducers are fed into a OR circuit which produces a local alarm. Each of the outputs is also fed to a multivibrator to produce a longer duration pulse which is then fed to a AND circuit which produces a second alarm, possibly at a remote location, such as an alarm monitoring station, if both transducers are activated during a predetermined time period.
U.S. Pat. No. 4,383,250 which issued on May 10, 1983 to Aaron Galvin discloses how one or two transducers may be utilized to differentiate between tampering.